This invention relates to a method for refining a molten metal bath, such as for producing steel. More particularly, the invention relates to controlling the nitrogen content of a molten metal bath refined in oxygen-type processes.
In the production of steel, for example, it is conventional practice to remove various impurities from molten metal, including decarburizing, to reduce the amount of carbon present. This is performed by introducing oxygen into a molten metal bath to produce a reaction between carbon dissolved in the molten metal bath and the oxygen to form volatile carbon oxides which are removed from the bath as gaseous reaction products. In addition to performing decarburization by the use of oxygen alone, it is known to use oxygen in combination with an inert gas. The inert gas improves the overall refining efficiency by reducing the partial pressure of CO and also serves as a means for agitating or stirring the bath. Since nitrogen is less costly than an inert gas, such as argon, it is widely used for cost reduction purposes. In decarburization practices of this general type, the gas may be introduced solely from tuyeres submerged in the bath, or from a lance directed onto or beneath the molten metal bath surface or, in addition, from tuyeres submerged in the bath. As a result of various ratios of nitrogen-to-oxygen used, the bath can become saturated with nitrogen. Also, in production facilities practicing transfer of molten metal between two or more ladles or vessels, additional nitrogen pickup occurs to the molten metal.
During this refining period, available metallic constituents from the molten steel may be oxidized to the slag. To recover metallics, such as chromium, to the metal, it is necessary to add a slag reductant, such as silicon or aluminum, for this purpose. An inert gas is generally used to stir the reductant within the bath for efficient reaction and to remove the nitrogen from the steel after the completion of decarburization, e.g., when the carbon content of the bath has been reduced to a selected level. It is known that after reduction, when the bath is deoxizided and oxygen is at a low level, nitrogen removal is more efficient. Oxygen is known to hinder the kinetics of nitrogen removal. For this purpose, an inert gas, generally argon, is introduced through tuyeres beneath the surface of the bath; however, the rate of flow of argon through tuyeres is restricted. High flow rates through tuyeres appear to increase vessel refractory wear and increase the cooling effect on the tuyeres, resulting in a buildup of "frozen metal" known to those skilled in the art as "knurdles" or "mushrooms" at the tuyere tip and decrease in efficiency. Also, during decarburization to prevent excessive nitrogen levels in the steel, an inert gas, such as argon, may be substituted for nitrogen during the blowing cycle at a specified time commonly termed the "nitrogen switch point." In view of such problems associated with removing nitrogen, it is conventional by those skilled in the art to control nitrogen by minimizing nitrogen pickup in the molten metal and not by removing nitrogen from high to acceptable levels.
U.S. Pat. No. 4,260,415, issued Apr. 7, 1981 to the Assignee of the present application, discloses, among other things, the use of argon through tuyeres to flush out or remove nitrogen from the bath to desired levels. A practice of using top-mixed gases including argon through a top lance in combination with bottom stirring action through a tuyere or plug is described in copending application Ser. No. 604,098, filed April 26, 1984, and assigned to a common Assignee.
It is, accordingly, a primary object of the present invention to provide a faster, more efficient practice for reducing the nitrogen content of the metal bath to the desired level upon the completion of the decarburization practice wherein nitrogen is substituted for an inert gas.
A more specific object of the invention is to reduce the nitrogen content of the metal bath to the desired level by the use of reduced amounts of inert gas, such as as argon.
Other more specific objects of the invention are to increase the efficiency of nitrogen removal from the metal bath and decrease the time required to supply the inert gas to the bath while minimizing wear on vessel refractory and reduce the buildup of knurdles which decrease the efficiency of decarburization.